Examining aluminium foils stopping potensial on <0.98c fast electrons

I got a Geiger-Muller counter, and a radioactive source.
The source is a Sr-90/Y-90 source, which emits highly energetic electrons, the highest energy is around 0.98c. The space between the source and counter is approximately 10 cm.
I am not operating in vacuum! The assignment I am doing is adding sheets of alufoil between the source and the counter, and observering the difference in counts per foil added.

I have the Kanaya-Okayama equation for penetration potensial in matter for electrons, but I dont know wether or not this can be used on electrons with momentum >30KeV.

I am observing that the absorbtion of electrons per foil that I add is not constant.
The decrease in counts per foil is not the same. At first, it is about 50, then 30, then around 5.
That is, each foil is about 14,5μm. Why is not the reductions in counts constant?
Is it relevant to think that this is because of the energyspectrum from the radioactive source?